Solid-state lighting is an attractive alternative to incandescent and fluorescent lighting systems because of its relatively higher efficiency, robustness and long life. However, many solid-state lighting systems utilize light-emitting diodes (LEDs) that require different drive circuitry than incandescent and fluorescent light emitters. LEDs are typically operated in current-control mode, in which the current through the LED is controllably set to particular values to achieve desired optical characteristics, such as brightness levels.
Manufacturing variations in electronic components may result in a distribution of electrical and optical parameters. For example, in the case of LEDs, there is generally a distribution in parameters such as forward voltage, light output power and wavelength. For LED-based lighting systems, particularly where such systems include arrays of LEDs, such variations result in the need for a system that can accommodate possible variations not only in the manufacturing distribution, but also that may arise from other sources, such as ambient or operational temperature variations, aging, or the like. This applies not only to light-emitting elements (LEEs), such as LEDs, but to all other active and passive components that may be in the system, e.g., to control the current to the LEDs or to power the entire system.
For example, consider the system shown in FIG. 1, which features one or more strings 160 of series-connected LEEs 110 and a current-control element (CCE) 120. The combination of the LEEs and the CCEs may be called a light-emitting array. A light-emitting array may include or consist essentially of one or more than one light-emitting string 160. The string voltage is the voltage of the sum of the forward voltages of the individual LEEs at the desired operating current added to the voltage dropped across CCE 120. In one example, for GaN-based blue LEDs, the forward voltage, for a fixed current, may be in the range of about 2.6 V to about 3.1 V, depending on variations in the LED fabrication process. Thus, depending on the distribution of forward voltage characteristics, the voltage across the 10 LEDs in FIG. 1 may range from about 26 V to about 31 V. For a light-emitting array with multiple strings in parallel, as shown in FIG. 1, a string with a relatively low string voltage will generally result in a relatively higher voltage dropped over CCE 120, whereas a string with a relatively higher string voltage will generally have a relatively lower voltage dropped over CCE 120. In the design of such systems the voltage of power supply 130 must be large enough to accommodate the highest possible string voltage within the manufacturing and operational distribution of the light-emitting array as well as the voltage supply.
Consider a relatively common case where the forward voltage of each LEE is nominally about 2.9 V and the nominal voltage drop across CCE 120 is about 2 V. For a string of 10 LEEs, the string voltage is then about 31 V. This sets the nominal value of voltage to be supplied to the light-emitting array at about 31 V.
Now consider the scenario where the string voltage is on the high end of the range, for example where the LEE forward voltage is about 3.1 V and the voltage drop across the 10 LEEs is about 31 V. In some embodiments, CCE 120 may require at least about 2 V to operate, so the light-emitting array requires a supplied voltage of 33 V, 2 V higher than the nominal amount. Next, consider the scenario where the string voltage is on the low end of the range, for example where the LEE forward voltage is about 2.7 V. The voltage drop across the 10 LEEs is then about 27 V.
In this situation, the voltage supply needs to be about 33 V to accommodate the high end of the LEE forward voltage distribution. However, in the nominal case, the voltage dropped across CCE 120 is 4 V and in the minimum forward voltage case it is 6 V. Thus, the power dissipated in CCE 120 in the nominal case is twice that of the maximum forward voltage case, and the power dissipated in CCE 120 in the minimum forward voltage case is three times that of the maximum forward voltage case. Even without accounting for other variations, such as in the voltage supply, CCE 120, or operational variations, it is clear that such a design may be optimized for efficiency in a narrow set of parameter ranges, but as a result of manufacturing and operational variations, may operate at significantly lower efficiencies. Further, the additional power dissipated in CE 120 results in additional heat, which may be difficult to remove and may also lead to thermal degradation and a reduction in lifetime and/or reliability.
One approach to mitigating this problem is reduce the manufacturing and/or operational variations that might be encountered, for example by sorting and binning LEEs, using higher precision components in the voltage supply and CCE, controlling the ambient temperature range, or the like. However, these approaches are undesirable because they are time consuming and expensive.
Accordingly there is a need for solutions that provide improved drive capability for LEE systems, in particular providing improved control of current through the LEEs as well as high efficiency.